Formation Procedure of Reaction Phases in Al Hot Dipping Process of Steel

This study investigated the nucleation and growth mechanism of reaction layers and phases of hot-dipped boron steel in pure Al at 690 °C for 0–120 s. In the case of a dipping time of 30 s, reaction nuclei of width 10–15 μm and height 10 μm were formed on the steel surface in the flow direction of the liquid Al. This reaction layer was formed as a mixture of θ (Fe4Al13) phase of several nm to 2 μm, θ and η (Fe2Al5) of several nm, a columnar η region, and a β (FeAl) region of 500 nm thickness at the steel interface. At the grain boundaries of ferrite, in contact with the η phase, κ (Fe3AlC) was formed. Using the calculated Fe-Al phase diagram, it was determined that when Fe was dissolved in liquid Al from the steel above 2.5 at% (0.6 wt%), the θ phase was formed. Although most of the θ phases continuously grew toward the liquid phase, the θ phase in contact with the steel was transformed into the η phase with minimal differences in composition due to the inter-diffusion of Al and Fe. It was therefore concluded that the η phase formed at the interface became a growth nucleus and grew in a columnar form toward the steel.